The present invention relates to apparatus and methods for measuring physical properties of samples when subjected to an external shear force.
Producing materials having specific required properties is steadily gaining in importance and, hence, the field of combinatorial chemistry, which generally refers to methods for creating collections of diverse materials or compounds, commonly known as libraries, is steadily increasing and has revolutionized the process of drug discovery. Combinatorial chemistry enables researchers to rapidly discover and optimize useful materials such as polymers, superconductors, magnetic materials, etc. In order to record the various properties of the materials obtained by combinatorial chemistry, it is necessary to precisely and efficiently determine the characteristics of the materials, preferably under varying environmental conditions. One important and useful characteristic is the behavior of a material, in particular of a polymer, when exposed to an externally-applied shear force. Instruments for measuring the response of fluids to applied shear are generally referred to as rheological instruments, which may be subdivided into indexers and rheometers. Indexers measure a quantity which is correlated with the rheological characteristics, but which is difficult to analyze in terms of intrinsic material properties. Although indexers can be assembled rapidly from commercially available components, the results of measurements carried out by means of such indexers are difficult to relate to results of measurements from other indexers, or to intrinsic material properties, without extensive calibration.
Rheometers, on the other hand, measure intrinsic material characteristics, giving them broad applicability. Such generality, however, comes at a price due to design costs and complexity which are dictated primarily by the need for well-defined static and dynamic test conditions. Moreover, these known rheometers require fairly large quantities of sample in the order of 500 mg so as to obtain the required accuracy in analyzing the samples. Such large quantity samples, however, are usually not provided by combinatorial synthesizing methods in which, generally, a large amount of differing samples of small quantity are produced.
Rheological measurements on sample materials, such as polymeric materials, are performed in their simplest geometry such that the sample is placed between two parallel plates of a design area separated by a gap of known distance, wherein the sample is sheared by applying a force to one of the plates while keeping the other plate fixed. This results in a displacement, i.e. a deformation of the sample confined between the plates, which can be characterized in terms of the shear stress and the shear strain. From these quantities and the dimensions of the sample, a shear modulus may be calculated. In general, the shear modulus is a function of the sample history, the shear strain and the strain rate. For polymeric materials, the temporal dependence of the shear modulus at constant stress typically exhibits four different regimes reflecting different relaxation mechanisms available to the polymer chain.
For sufficiently small deformations, most polymers exhibit linear viscoelastic behavior in which the shear modulus is independent of the shear strain. Theories of polymer dynamics generally explain the response of a chain in terms of normal modes, each having a characteristic frequency. The linear viscoelastic theory gives, then, the response of the material as a function of shear history. In measuring the mechanical property of a sample material, the sample is subjected to a varying force, e.g. a sinusoidal-varying force, and the resulting deformation, i.e. the response of the sample is observed. The frequency response of the sample may then be analyzed in accordance with viscoelastic theories to obtain information on the required characteristic of the material.
In order to perform these measurements with a high degree of accuracy, the rheological apparatus must be capable of producing a well-defined displacement within a specified frequency range. Additionally, since each frequency corresponds to probing the response of the sample material at a particular relaxation time, such measurements take a relatively long time period when the relaxation mechanism of the samples requires the employment of low frequencies. Hence, measuring a plurality of samples which may be produced by combinatorial chemistry is a very time-consuming and therefore very expensive procedure.
Moreover, the performance of accurate measurements requires the application of suitable sensor elements for detecting the shear stress in the samples. In order to obtain meaningful experimental results, the sensor elements have to be suitably designed so as to reflect the response of the sample to the applied shear strain without any interference or at least minor interference of the sensor element.
In view of the above-mentioned problems, it is an object of the present invention to provide apparatus and methods for rheological measurements which are capable of producing reliable measurement results for a plurality of small quantity samples within a short time period.
According to one aspect of the present invention, there is provided a miniature rheometer for analyzing a small quantity sample of a material of interest, wherein the rheometer comprises a first plate and a second plate, forming a pair of plates having a known geometry for confining the sample between the plates, said sample having a volume of 200 microliters or less; an adjusting device for adjusting the separation of the plates; an actuating element mechanically coupled to the first plate, which produces a shear strain within the sample by generating a defined small-scale relative motion of the first and second plates; a sensing element which outputs a position signal indicative for a displacement of at least one of the first and second plates; and a feedback circuit for providing force rebalance of the force, applied to the sample by the small-scale relative motion of the first and second plates, on the basis of the position signal, wherein an amount of force rebalance is a measure for the stress within the sample.
The miniature rheometer according to the present invention is suitably adapted to characterize combinatorial materials. In contrast to known instruments which generally require large sample volumes, typically 10-100 times the quantity produced by current combinatorial synthetic approaches, the present invention merely requires sample quantities having a volume of 200 microliters or less, or preferably 10-50 microliters, which are easily obtained by combinatorial synthesizing methods. Since measurements on such small sample volumes requires an accurate response of the rheometer on a corresponding small length scale, the present invention provides a sensing element and a feedback circuit which provide for force rebalance of the force that is applied to the sample in order to avoid the undesired inherent displacement of conventional force sensor elements due to shear forces exerted by the sample on the force sensor. Accordingly, the force balance may be controlled such that the present miniature rheometer exhibits an extremely high effective stiffness with respect to the sample, which in turn insures accurate measurement results, even at very small displacements.
In further embodiments, the position signal which is input into the feedback circuitry so as to adjust the force rebalance is obtained from the sensing element which may comprise a deformation-sensing element, a encoder means, or any other appropriate means suitable to determine the location of the plates with a spatial resolution that is substantially smaller than a minimal displacement of the plates as required for the desired measurement accuracy. The rheometer may comprise a sensor-actuating element that is mechanically coupled to the second plate so as to maintain the second plate at a predefined position upon reception of a driving signal from the feedback circuit. Alternatively, the actuating element may be driven from the feedback circuit such that the first plate maintains a desired displacement, wherein the second plate may be a fixed plate or may be kept at a fixed position.
In a further embodiment, the actuating element used as a shear strain producing means comprises a piezo-electric actuator so as to produce the small-scale relative motion. This allows the miniature rheometer to create small relative displacements of the plates while insuring easy control and configuration of the actuator.
In a further embodiment, the plates are disposable plates. The employment of disposable plates may considerably facilitate sample preparation and sample replacement after completion of a measurement run.
Advantageously, the deformation sensing element is coupled to the actuating element or the sensor-actuating element or to both and detects a deformation of at least a portion of the respective actuating element. The deformation sensing element measures the amount of deformation generated by the shear force applied to the sample. In this manner, the signal from the deformation sensing element may either be directly used as a measure for the shear force, or it may be supplied to the feedback circuit which adjusts the force exerted on the deformation sensing element by the actuating element or the sensor-actuating element so as to return the deformation sensing element to a predefined location, e.g. the undeflected state of the deformation sensing element.
In a further embodiment, two or more miniature rheometers may be arranged so as to form a parallel rheometer for simultaneously measuring two or more small quantity samples. To this purpose, a common control unit is provided which controls the shear strain producing means and the force sensors of the two or more miniature rheometers.
According to a second aspect of the present invention, there is provided a parallel rheometer for simultaneously analyzing material characteristics of two. or more samples, wherein the parallel rheometer comprises first and second plates respectively having regions for receiving and confining said two or more samples, the first and second plates being moveable relative to each other; an actuator adapted to move the first and second plates relatively to each other for producing a shear strain within each sample; and at least one sensor associated with each region for simultaneously detecting shear stress within each sample.
As previously stated, standard rheological measurements often characterize materials according to their frequency response to an applied oscillatory shear force. Here, the frequencies of interest set the minimum measurement time required, which is typically three or four times the reciprocal of the frequency. Thus, by allowing the simultaneous measurement of a large number of samples by means of a parallel rheometer according to the present invention, a quick screening of a plurality of material samples (such as those produced by combinatorial synthetic approaches) is feasible. Moreover, according to the present invention, the minimum sample volume may be kept smaller than in known single-channel rheometers so as to permit measurements as a function of environmental conditions to use much faster condition change rates than are possible with large samples. To this end, the parallel rheometer as well as the miniature rheometer may comprise means for applying varying environmental conditions. Preferably, the environmental conditions to be varied, individually or simultaneously in any combination, at least include temperature, pressure at a fixed gas composition, composition of a gas atmosphere surrounding the sample, electric field, magnetic field, and time of application of one or more of the preceding quantities when adjusted to respective predetermined values. The means may be designed so as to allow the variation of the environmental conditions individually for each sample and/or simultaneously for a group of samples.
In a further embodiment, the shear stress detector comprises a micromachined sensor element at each sample position. This allows mass production of nearly identical sensor elements at low cost, wherein the required sample volume may easily be maintained relatively small due to the reduced sensor mass and increased sensitivity provided by micromachined devices.
According to a third aspect of the present invention, there is provided a rheometer, comprising: a pair of plates spaced apart from each other by a defined distance for receiving and confining a sample therebetween, an adjusting means which adjusts the distance between the plates, a driving means coupled to at least one of the plates, which generates a relative motion between the plates without changing the distance, and a shear stress sensor, the shear stress sensor comprising a stress-sensing material of a defined stress-optic coefficient indicating one of birefringence and retardation of linearly polarized light passing the stress-sensing material, as a function of applied stress/unit path length.
The shear stress detector comprises a stress-optical sensor element which is insensitive to electric and magnetic fields, and thus allows to analyze samples within such fields without creating electrical noise.
According to a fourth aspect of the present invention, there is provided a sensor element for outputting a signal in response to a mechanical deformation applied to the sensor element, wherein the sensor element comprises: a sample plate arranged within an opening of a substrate; at least two tethers, one of each tether being attached to the sample plate, the other end of each tether being attached to the substrate so as to support the sample plate; a piezo-resistive portion in each of the tethers; and a wiring line formed on the tethers and the substrate, connecting each piezo-resistive portion with a corresponding contact pad formed on the substrate, wherein the piezo-resistive portion of one of said at least two tethers is adapted to generate a maximum change of its internal resistance when a shear force is applied to the sample plate, and wherein the piezo-resistive portion of the other one of said at least two tethers is adapted to generate a maximum change of its internal resistance when a force normal to the sample plate is applied.
As is generally known, shearing a viscoelastic material also generates a force along the shear gradient direction. Such forces are potentially of either sign and can be comparable in magnitude to the shear force. They can strongly affect the macroscopic flow properties of a material. At a minimum, rheometers must be sufficiently stiff in the shear gradient direction so that shear actuation results in as near a pure shear field as possible, and that any strain determination measures only the shear strain. By means of the sensor element provided according to the third aspect of the present invention, a shear force and a normal force applied to a sample can be detected simultaneously. This improves the accuracy of the measurement results due to separation of the total instrumental response into shear and normal force components and provides a more comprehensive picture of the response of the sample to applied shear.
This sensor element is particularly advantageous when used in combination with the above parallel rheometer.
Further advantages and objects of the present invention follow from the dependent claims and the detailed description of the preferred embodiments